Sizing hydraulic motor

I have a basic understanding of hydraulic stuff and I have bult several pieces of equipment that used hydraulics. However I have not done anything that used a hydraulic motor.

I am working on a trailer mounted post driver that,,, well the plan is to have a 12-16hp enginge driving a hydraulic pump that will drive a motor for a screw auger for drilling a 4 inch hole about 3 1/2 feet deep, then drive a wood post down the hole. Much like the truck setups used by highway crews for guard rails. Looking to have it set solid and verticle in one shot.

We will be putting in a lot of fence over a long period of time. I could just use the PHD but it is a pain for more than a few holes (keeping them straight up and down). The thought is this would be much easier and quicker. I was also thinking if I could size one that would do up to 6 inch, we could do the corners with the same setup.

I just am not sure how to figure the sizing of the pump and motor (more motor than pump). Do not have to dig through anything crazy, just clay sand mix with some stone. So anyone know how / where one would start to figure this out?

Just a suggestion. Give Surplus Center Technicial Division and see what they can contribute. They do Know their Hydraulics. 1-800-488-3407 24/7days a week. Hope this helps.
LOU

I concur with Lou, surpluscenter.com is the address, and their catalog has a page with the various formulas for calculating what you want. Has to do with flow rate of the pump, displacement of the motor, and efficiency of pump and motor.

Did you look into a vibrating post pounder attachment for a skidsteer or frnt end loader?
Can't be beat IMO

Bison, I've been looking into making a post pounder. What is vibrating one consist of. Never heard of one.

Errin, I was researching it last year. Talked to and emailed surpluscenter. Once I asked them what size would work best with my tractor, they clammed up and said they couldn't help with design.

I've looked at a number of the pto driven ones and the hydraulic ones you can buy or rent.

I was probably trying to go to big with too much torque.

Most I've seen use 8 to 10 gpm pumps and the motors just have a 1/2" shaft so thats not a big motor and not much hp and torque. The speed is proportional to the gpm you pump through a motor. The torque is proportional to the PSI you are using.

Most that I have looked at figure out to be about 50 to 300 auger rpm. The TSC pto auger is around a 3.5 to 1 drive ratio as I recall without digging out my notes with 540 rpm. (I turned one in the store) Figures out at 150 auger rpm but most people run them at idle. 50 rpm would be plenty. The power pole augers the power co. uses may be slower that that. Too fast is an accident waiting to happen. I have an idea what I would buy but need a valve and maybe a flow divider and then find the time to build a 3 point mount for it. Now I think I'd rather build a pile driver to drive posts. I already have a 2 man auger.

If you ask surpluscenter to design it for you, of course, they will clam up. Request a catalog, or search hydraulic formulas, on their site, and you can design your own system, if you can do simple algebra.